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Interpretive Summary: Large-scale production of the rust fungus, Puccinia jaceae var. solstitialis, was managed successfully in both a greenhouse environment and in field plots. The rust was approved in 2003, for classical biological control of yellow starthistle, Centaurea solstitialis, in California. There were two important questions about introduction of the rust fungus, Puccinia jaceae, for biological control of yellow starthistle (YST) in California. The first question was whether all of the YST in California was susceptible. Inoculations were made of YST collected from all over California, so all the different habitats were included in the test. All of the YST from California was susceptible to P. jaceae following greenhouse inoculations. The second question was how to get enough inoculum to use in a large-scale release program. This was a unique problem, because P. jaceae needs to be grown on living plants; it can’t be grown on agar media. Production of inoculum was accomplished by inoculating plants, giving them dew, growing them in a greenhouse, and then harvesting spores (inoculum) with a special vacuum harvester. As a result, over 64 grams of spores were collected, and those not used were stored in a very cold (minus 80 C) freezer.

Technical Abstract: Urediniospores produced in quarantine at Ft Detrick, Maryland, were tested for purity before removal from containment and transport to California. Greenhouse production of this obligate pathogen in California was required to meet inoculum needs in large-scale release programs in California and, possibly, other western states. Over 64 grams of spores were produced in the greenhouse from 2003-2006. Spore yield in the greenhouse varied by season, with peak production in early spring through early summer. Large-scale urediniospore collection was successful also from an outdoor field plot in Davis, California. Finally, 62 field accessions, representing the range of habitats for yellow starthistle, were tested in the greenhouse and determined to be equally susceptible to the rust. Our results show that large-scale production of an obligate pathogenic biological control agent is possible to support statewide release and research programs. Findings suggest also that all YST in CA is likely susceptible to the particular isolate used in the release.